Inhibition of catalyzed oxidation of carbon-carbon composites

Abstract: This invention relates to a composition and method of inhibiting catalyzed oxidation of carbon-carbon composites. The invention includes compositions, capable of impregnating carbon-carbon composites, comprising an aqueous mixture of (a) phosphoric acid, (b) a metal phosphate, and (c) a penetration and retention improving amount of a compatible wetting agent selected from the group consisting of polyols, alkoxylated monohydric alcohols, and mixtures thereof. The invention also includes a method of inhibiting catalyzed oxidation of carbon-carbon composites comprising the steps of treating a carbon-carbon composite with the catalyzed oxidation inhibiting aqueous mixture, and heating the treated carbon-carbon composite to a temperature sufficient to remove water. Articles, including friction brakes, made by this method are also part of this invention. Treated brake lugs, and other exposed areas of aircraft braking systems have improved resistance to catalyzed oxidation.

Description: Technical Field of the Invention This invention relates to a method and composition useful in inhibiting catalyzed oxidation of carbon-carbon composites. Background of the Invention Many aircraft utilize a stack of carbon composite discs in brakes that can absorb large amounts of kinetic energy required to stop the aircraft during landing or in the event of a rejected take-off. During some of the stops, the carbon is heated to sufficiently high temperatures that surfaces exposed to air will oxidize. Carbon composites with thermal and mechanical properties required for specific brake designs have been prepared. However, these composites have had residual open porosities (typically 5% to 10%) which permit internal oxidation. The internal oxidation weakens material in and around the brake rotor lugs or stator slots. These areas transmit the torque during braking. One simple, low-cost method of minimizing loss of strength and structural integrity is application of phosphoric acid to non-wear surfaces of brake discs, followed by baking to 650°C. This method inhibits normal oxidation of carbon sufficiently for many applications, including aircraft brakes. However, some commercial transport brakes have suffered crushing in the lugs or stator slots. The damage has been associated with visible surface oxidation and oxidation enlargement of cracks around fibers. The enlargement occurs at depths up to 1.27 cm (0.5 inch) beneath the exposed surfaces. Potassium or sodium has been identified in the severely oxidized regions. Alkali and alkaline earth elements are well known to catalyze carbon oxidation. Catalyzed oxidation is carbon oxidation that is accelerated by the presence of contaminating materials. These contaminating materials come into contact with the brake from clearing and de-icing chemicals used on aircraft. Sodium can originate from salt deposits left from seawater or sea spray. These liquids, and other deicers or cleaners containing K or Na, can penetrate the porous carbon discs leaving catalytic deposits within the pores. When such contamination occurs, the rate of carbon loss by oxidation can be increased by one to two orders of magnitude. There is a need to provide protection against such catalyzed oxidation. McKee points out that phosphates can deactivate catalytic impurities in carbon by converting them to inactive, stable phosphates (D. W. McKee, Chemistry and Physics of Carbon 16 , P.l. Walker and P.A. Thrower eds., Marcel Dekker, 1981, p. 30.) Woodburn and Lynch (US-A-2,685,539) describe several ways of impregnating pores in carbon or graphite with aluminum phosphate. They specified compositions having a molar ratio of Al 2 O 3 :P 2 O 5 between 0.2:1 and 0.8:1. Application methods included brushing, spraying or soaking in solutions, including Al(H 2 PO 4 ) 3 dissolved in aqueous HCl. U. S.-A-4,439,491, issued to Wilson, relates to carbon or graphite protected against oxidation by application of a solution comprising monoammonium phosphate, zinc orthophosphate, phosphoric acid, boric acid, cupric oxide, and wetting agent in water. U.S.-A-4,837,073, issued to McAllister et al., relates to a barrier coating and penetrant providing oxidation protection for carbon-carbon materials. The method involves penetrating a barrier coating for carbon-carbon materials with an oxygen inhibitor. US-A-4,865,646 discloses a formula for a fountain solution comprising water, alkali pyrophosphates, phosphoric acid, ethylene and propylene glycol and Surfynol® 440 (alkoxylated acetylenic polyol). It is desirable to have a simple, effective method and composition to inhibit catalyzed oxidation of carbon. Summary of the Invention This invention relates to a composition and method of inhibiting catalyzed oxidation of carbon-carbon composites. The invention includes compositions, capable of impregnating and inhibiting catalyzed oxidation of carbon-carbon composites, consisting essentially of a phosphate catalyzed oxidation inhibiting mixture, and a penetration and retention improving amount of a compatible wetting agent selected from the group consisting of polyols, alkoxylated monohydric alcohols, and mixtures thereof. The invention also includes the method of inhibiting catalyzed oxidation of carbon-carbon composites comprising the steps of treating a porous carbon-carbon composite with a catalyzed oxidation inhibiting amount of an aqueous mixture comprising the above-described composition and heating the coated carbon-carbon composite to a temperature sufficient to remove water. Articles, including friction brakes, can be made by this method. The compositions and method show improved penetration and retention of the catalyzed oxidation inhibitors. Description of the Preferred Embodiments As used in the specification and claims, the term "retention" refers to preventing the migration of the liquid impregnant from the pores of the carbon-carbon deposit to the surface thereof upon heating. This invention is particularly useful for aircraft brakes which operate at temperatures up to 700°C during normal service. It is in the lower temperature range, e.g. above 400°C, that the effect of catalytic oxidation is most severe in allowing more rapid degradation of a carbon body's strength by oxidation internally from air diffusing into the pores. Application of this invention provides a means for significantly extending the service life of carbons by preventing catalyst-accelerated, internal oxidation. In order to protect carbon from rapid oxidation by catalysis from Na, K, or other metals in contaminating liquids encountered during service, metal phosphate deposits should be distributed uniformly throughout the pores (e.g. with not more than ∼1 mm separation between deposits) to a depth sufficient to cover the range affected by O 2 diffusing in from exposed surfaces (∼1 cm). The process described here has provided such deposits when the carbon is heated in service to the range 400°C to 700°C (but below ∼1040°C). Carbon-carbon composites are generally prepared from carbon preforms. Carbon preforms are made of carbon fibers, which may be formed from preoxidized acrylonitrile resin. In one embodiment, these fibers can be layered together to form a shape, such as a friction brake. The shape is heated and infiltrated with methane or another pyrolyzable carbon source to form the carbon-carbon composites. Carbon-carbon composites and methods of their manufacture are known to those in the art. In one embodiment, the carbon-carbon composite has a density from 1.6 to 1.9 g/cm 3 . A particularly useful carbon-carbon composite has a density of 1.7 g/cm 3 . The carbon-carbon composites are impregnated with an aqueous mixture of (a) phosphoric acid, (b) a metal phosphate, and (c) a penetration and retention improving amount of a compatible wetting agent selected from the group consisting of polyols, alkoxylated monohydric alcohols, and mixtures thereof. The wetting agents used in this invention are compatible with the aqueous mixture of phosphoric acid and metal phosphate. In one embodiment, the aqueous composition is free of boric acid and metal borates. In another embodiment, the aqueous composition is free of hydrochloric acid. The aqueous composition includes a metal phosphate. Illustrative examples of useful metal phosphates include manganese, zinc and aluminum phosphates. In one embodiment, the metal phosphate is aluminum phosphate, preferably mono-aluminum phosphate (Al(H 2 PO 4 ) 3 ). In the aqueous compositions, the molar ratio of aluminum to phosphorus is generally from 0.2 up to 0.8 (aluminum) to one (phosphorus). The aqueous composition generally contains from 40%, or 45%, or 50% up to 70%, or to 65% by weight water. The phosphoric acid (A) is present in an amount from 50%, or 55%, or 60% up to 75% or to 70% by weight of combination of (A) and (B). The metal phosphate (B) is present in an amount from 25%, or 30%, up to 50%, or to 45%, or to 40% by weight of the combination of (A) and (B). The aqueous compositions of the present invention additionally contain a compatible wetting agent. The wetting agent is selected from the group consisting of polyols, alkoxylated monohydric alcohols, and mixtures thereof. The wetting agent is present in an amount sufficient to improve the penetration and retention of the metal phosphate and phosphoric acid. The wetting agent is typically present in an amount from 0.3%, or 0.5%, or 0.75% up to 3%, or to 2%, or to 1.5% by weight of the combination of the phosphoric acid (A) and the metal phosphate (B). In one embodiment, the wetting agent is present in an amount from 0.5% to 1% by weight of the combination of (A) and (B). The wetting agent is generally present in the aqueous mixture in an amount from 0.1%, or 0.3% up to 2%, or to 1% by weight of the aqueous mixture. When the wetting agent is a polyol, it generally contains two, three, or four hydroxyl groups, preferably two hydroxyl groups. In one embodiment, the polyol is alkoxylated. In another embodiment, the polyol is an acetylenic polyol. The acetylenic polyol may be branched. Examples of acetylenic polyols include dimethylhexynol, dimethyloctynediol, and tetramethyldecynediol. Acetylenic polyols are available commercially from Air Products & Chemicals, Inc. under the tradename Surfynol®. An example of a useful acetylenic polyol is Surfynol® 104. The acetylenic polyol may also be alkoxylated. These materials are generally prepared by treating an acetylenic polyol with an epoxide, such as ethylene oxide, propylene oxide, etc. An example of a useful alkoxylated acetylenic polyol is Surfynol® 440. The wetting agent may also be an alkoxylated monohydric alcohol. The alkoxylated monohydric alcohols are generally prepared by reacting a monohydric alcohol with an epoxide, such as those epoxides described above. In one embodiment the alcohol contains from 8, or 10 up to 24, or to 18 carbon atoms. The alkoxylated monohydric alcohol may be an alkoxylated linear alcohol. An example of a useful alkoxylated alcohol is Polytergent® SL-62 available commercially from Olin Corporation. In one embodiment, the invention relates to a composition consisting essentially of a phosphate oxidation inhibiting mixture, and a penetration and retention improving amount of a compatible wetting agent selected from the group consisting of polyols, alkoxylated monohydric alcohols, and mixtures thereof. The phosphate oxidation inhibiting mixtures are those which inhibit catalyzed oxidation of the carbon-carbon composites. These mixtures include metal phosphates and phosphoric acid as described above. The wetting agents also are described above. The invention also relates to a method of inhibiting catalyzed oxidation of carbon-carbon composites comprising the steps of treating a carbon-carbon composite with a catalyzed oxidation inhibiting amount of an aqueous mixture comprising a metal phosphate, phosphoric acid, and a compatible wetting agent selected from the group consisting of polyols, alkoxylated monohydric alcohols, and mixtures thereof and heating the treated carbon-carbon composite to a temperature sufficient to remove water. In one embodiment, the aqueous mixture is applied to regions exposed to oxidation, such as brake stators and lugs. The mixture is prepared by blending the components of the mixture. In one embodiment, the components are mixed ultrasonically. Typically the method occurs at atmospheric pressure. The aqueous mixture may be applied by any means known to those in the art, including painting, dipping, spraying, etc. The treated carbon-carbon composites are then heated to remove water. In one embodiment, the heating is done in an inert atmosphere, such as a nitrogen atmosphere. The treated carbon-carbon composites are generally heated to a temperature from 350°C, or 450°C or 550°C up to 900°C, or to 750°C, or to 650°C. In one embodiment, the treated carbon-carbon composites are heated to 600°C to 700 °C. The invention also includes articles made by the above described process. In one embodiment, the invention relates to friction brakes made by the above method. The following examples relate to the aqueous compositions and methods of the present invention. Unless otherwise indicated, throughout the specification and claims, parts are parts by weight, temperatures are degrees Celsius, and pressure is atmospheric pressure. Example 1 An aqueous mixture is prepared by mixing 60 parts of a 50% by weight of an aqueous mixture of mono-aluminum phosphate available from Stauffer Chemical, 20 parts of 85% by weight phosphoric acid aqueous solution 20 parts of water, and 0.5 parts of alkoxylated tetramethyldecynediol (Surfynol® 440). The aqueous mixture is ultrasonically agitated for 30 minutes. The mixture is painted onto a carbon-carbon composite so that pores near the painted surface are impregnated, and the carbon-carbon composite thus treated is heated to 650°C under nitrogen Example 2 The procedure of Example 1 is followed except 0.5 parts of tetramethyldecynediol (Surfynol® 104) is used in place of alkoxylated tetramethyldecynediol. Example 3 The procedure of Example 1 is followed except 0.5 parts of an alkoxylated linear alcohol (Polytergent® SL-62) is used in place of alkoxylated tetramethyldecynediol. Experimental Coupons, approximately 2.5 cm by 2.5 cm by 0.6 cm, are cut from brake material, manufactured by densification of a fiber preform by chemical vapor infiltration/deposition of carbon (CVI-C). In one embodiment, the carbon is further subjected to a heat treatment at about 2200°C. Oxidation tests are conducted by comparing weight loss in a lot of 12 to 20 coupons during isothermal exposures in flowing air at temperatures between about 480°C and 705°C. The times are shown below in Table 1. The time and temperature profiles simulate aircraft landings. The longer times (except for overnight holds) are broken into two or four increments to allow shifting of coupon locations to compensate for temperature gradients within the furnace. Minutes at Temperature in Oxidation Tests Furnace Test Temperature (°C) 482°C 538°C 593°C 649°C 704°C 1. First 600 Landings 900 64 104 -- -- 2. + Next 600 Landings 960 226 52 70 -- 3. + Next 400 Landings 1260 382 126 30 27 The three conditions equal a total of 1600 landings. Coupons are placed on Al 2 O 3 supports and inserted into the preheated furnace. Alter removal, weights are subtracted from the as-inhibited weight, obtained after heating 5-6 minutes at 593°C in air, and divided by the uninhibited dry carbon weight to compare carbon weight loss after successive "landings". Following an initial oxidation equivalent to "1200 landings" to simulate initial service, contamination is simulated by soaking specimens for about 5 minutes in seawater containing 10.8 mg/ml Na, 1.46 mg/ml Mg, 0.43 mg/ml K and 0.33 mg/ml Ca (all oxidation catalysts). In some tests, coupons are soaked instead in a solution of runway deicer. The runway deicer (E36) contains about 51% potassium acetate and is manufactured by Cryotech Deicing Technology. Samples for testing are supplied by Chevron Chemical Company. After contamination, the coupons are first dried and then are heated for 5 minutes at 593°C; the weight increase (∼0.1% of the carbon weight) is added to the previous weight from which later weights were subtracted to calculate percent carbon lost. Coupons are treated with St-A, mono-aluminum phosphate (Stauffer Chemical Co., 50% Al(H 2 PO 4 ) 3 ), or M-1, the product of Example 1 and are exposed to oxidation as described above. Table 2 contains data comparing coupons inhibited with aluminum phosphate and coupons treated with the compositions of the present invention. Table 2 contains data showing the weight change associated with the inhibitor treatment, pre-oxidation equivalent to 1200 landings, and seawater treatment of the coupons, as well as, the weight loss associated with oxidation equivalent to 600 and 1600 landings. Note that inhibition was more difficult in carbon without a final heat treatment, where weight increase after inhibition was less. The M-1 of example 1 was more effective than St-A even though less inhibitor was added. Treatment Weight Change , % % Wt. Inhibitor + Inhibitor 1200 Ldgs + Seawater 600 Ldgs 1600 Ldgs 2-Propanol Wash + 149°C 1-Hour Dry A -- St-A +1.83 -0.12 +0.05 -1.89 -8.05 -- M-1 +1.19 -0.16 +0.06 -0.98 -4.53 B -- St-A +3.09 -0.05 +0.10 -0.41 -1.60 -- M-1 +2.40 -0.20 +0.16 -0.37 -1.22 HNO 3 Soak + 149°C 1-Hour Dry C +0.02 St-A +2.27 -0.12 +0.06 -1.16 -3.80 +0.02 M-1 +1.13 -0.16 +0.06 -0.81 -2.90 D +0.04 St-A +4.57 -0.08 +0.07 -0.19 -0.55 +0.04 M-1 +2.45 -0.21 +0.05 -0.29 -0.52 649°C Bake In N 2 E +0.002 St-A +1.93 -0.15 +0.07 -1.76 -6.03 +0.007 M-1 +1.63 -0.14 +0.09 -1.04 -3.71 F +0.055 St-A +2.31 -0.06 +0.15 -0.72 -2.95 +0.062 M-1 +2.22 -0.18 +0.17 -0.29 -0.95 These data show that carbon treated with the compositions and methods of the present invention are better protected against catalyzed oxidation, e.g. have less weight loss, compared to carbon treated with aluminum phosphate. Table 3 contains data showing the effectiveness of differing levels of wetting agent on carbon weight loss. Surfact. % in M-1 Wt. Change, % +Inhib. 1200 Ldgs +Seawater 600 Ldgs 1600 Ldgs 0.3 +2.12 -0.11 +0.11 -1.59 -10.24 0.5 +1.47 -0.12 +0.07 -1.21 -6.75 0.5 +0.97 -0.18 +0.09 -3.12 -17.27 0.5 +1.39 -0.13 +0.10 -1.16 -7.39 0.5 +0.81 -0.14 +0.08 -1.67 -10.08 0.5 +2.23 -0.12 +0.14 -1.14 -7.49 0.5 +2.07 -0.15 +0.09 -1.88 -12.76 1.0 +2.06 -0.14 +0.10 -1.21 -7.29 1.0 +1.79 -0.14 +0.08 -1.56 -9.66 As a test of the effectiveness of the preferred composition and processing method against oxidation catalyzed by potassium in runway deicer, coupons are painted with either phosphoric acid or "M-1", described above. The coupons are baked together in an oven heated to 649°C. They are then preoxidized in cycles equivalent to either 1200 landings or 600 landings, and are contaminated by soaking for 5 minutes in solutions with increasing amounts of runway deicer E36 added to deionized water. In these tests, the "landings" after contamination are simulated by oxidation in flowing air in a furnace which is cooled from a maximum temperature at the temperatures and for times shown in Table 4. COOLING PROFITS FOR "LANDINGS" SIMULATION OF CARBON OXIDATION Temp., ° C Time, Minutes, For Cooling At A Uniform Rate In "Landings" From To 1-600 601-800 801-1000 1001-1200 1201-1400 1400-1600 704 677 6 8 14 677 649 6 10 7 6 6 649 621 15 7 7 7 7 7 621 593 24 7 7 8 8 30 593 566 22 8 8 8 36 66 566 538 27 12 20 51 71 77 538 510 33 24 54 76 100 140 510 482 56 108 194 256 252 236 482 454 641 261 209 153 188 246 454 427 553 182 235 352 414 500 Table 5 contains data of the treatment levels of inhibitor and deicer, as well as the weight gain associated with those treatments. Table 6 contains data showing weight loss under catalyzed oxidation conditions. EFFECT OF E36 POTASSIUM ACETATE RUNWAY DEICER ON CARBON OXIDATION %Surf. in M-1 Weight Change Per Cent E36 % +Inhib. & 593°C Total "landings" +E36 & 300°C 600 1200 "M-1" Inhibited 1.0 30 +2.02 -0.06 -0.22 +0.29 1.0 30 +2.30 -0.05 -0.20 +0.42 1.0 10 +2.25 -0.06 -0.21 +0.11 1.0 10 +1.54 -0.06 -0.21 +0.09 1.0 3 +2.28 -0.05 NA +0.03 1.0 3 +1.26 -0.06 NA +0.03 0.5 3 +1.92 -0.05 NA +0.01 0.5 3 +1.97 -0.05 NA +0.02 H 3 PO 4 Inhibited -- 30 +0.04 -0.06 -0.58 +0.68 -- 30 +0.04 -0.05 -0.52 +1.03 -- 10 +0.03 -0.04 -0.51 +0.26 -- 10 +0.03 -0.05 -0.54 +0.20 -- 3 +0.41 -0.06 NA +0.05 -- 3 +0.12 -0.04 NA +0.05 -- 1 +0.15 -0.06 NA +0.00 -- 1 +0.24 -0.06 NA +0.00 % Surf. in M-1 Weight Change Per Cent %E36 Deicer Total "Landings" 601-800 801-1000 1001-1200 1201-1400 1401-1600 1.0 30 NA NA NA -5.32 -7.56 1.0 30 NA NA NA -5.17 -7.58 1.0 10 NA NA NA -2.35 -3.84 1.0 10 NA NA NA -2.89 -4.65 1.0 3 -0.09 -0.11 -0.18 -0.27 -0.53 1.0 3 -0.19 -0.23 -0.37 -0.58 -1.13 0.5 3 -0.09 -0.11 -0.18 -0.30 -0.59 0.5 3 -0.08 -0.10 -0.15 -0.25 -0.53 H 3 PO 4 Inhibited -- 30 NA NA NA -100 -- 30 NA NA NA -100 -- 10 NA NA NA -100 -- 10 NA NA NA -100 -- 3 -2.71 -4.57 -8.87 -13.08 -19.12 -- 3 -11.25 -16.45 -26.39 -34.80 -46.01 -- 1 -0.80 -1.40 -2.98 -4.69 -7.54 -- 1 -0.13 -0.21 -0.53 -0.96 -2.28 As shown, the coupons oxidize more rapidly with increasing amounts of deicer contamination, and the inhibited coupons are effectively protected. Some protection results if metal phosphate is formed from contamination of H 3 PO 4 -treated coupons resulting in a greater weight increase during bake.

Claims: A composition consisting essentially of a phosphate catalyzed oxidation inhibiting mixture, and a penetration and retention improving amount of a compatible wetting agent selected from the group consisting of polyols, alkoxylated monohydric alcohols, and mixtures thereof. The composition of claim 1 wherein the phosphate catalyzed oxidation inhibiting mixture comprises a metal phosphate and phosphoric acid. The composition of claim 2 wherein the metal phosphate is monoaluminum phosphate. The composition of claim 3 wherein the molar ratio of aluminum to phosphorus is from 0.2:1 to 0.8:1. The composition of claim 1 wherein the wetting agent is an acetylenic polyol or an alkoxylated acetylenic polyol. The composition of claim 1 wherein the wetting agent is tetramethyldecynediol or alkoxylated tetramethyldecynediol. The composition of claim 1 wherein the wetting agent is an alkoxylated monohydric alcohol prepared by reacting a monohydric alcohol having 8 to 24 carbon atoms with an epoxide. The composition of claim 1 wherein the wetting agent is present in an amount from 0.1% to 2% by weight. The composition of claim 1 wherein the wetting agent is present in an amount from 0.5% to 1% by weight. A method of inhibiting catalyzed oxidation of carbon-carbon composites comprising the steps of treating a carbon-carbon composite with a catalyzed oxidation inhibiting amount of an aqueous mixture comprising the composition as defined in anyone of claims 1 to 9 and heating the treated carbon-carbon composite to a temperature sufficient to remove water. The method of claim 10 wherein the process occurs at atmospheric pressure. The method of claim 10 wherein the treated carbon-carbon composite is heated to a temperature from 350 °C up to 900 °C. The method of claim 10 wherein the treated carbon-carbon composite is heated to a temperature from 600 °C to 700 °C.

Inventors: DIETZ ROBERT PAUL, STOVER EDWARD ROY, DIETZ, ROBERT PAUL, STOVER, EDWARD ROY
Applicants: GOODRICH CO B F, THE B.F. GOODRICH COMPANY
Application year: 1994
Main classification: C04B41
IPC codes: F16D69, C04B41